The technical background is for example defined in the technical report of the 3rd Generation Partnership Project (3GPP) TR 25.896 V0.3.1 (2003-05).
Hybrid ARQ (H-ARQ)
H-ARQ is an implicite link adaptation technique. In H-ARQ, link layer acknowledgements are used for re-transmission decisions. There are different schemes for implementing H-ARQ-Chase combining, rate-compatible punctured turbo-codes and incremental redundancy. Incremental redundancy or H-ARQ-type-II is another implementation of the H-ARQ technique wherein instead of sending simple repeats of the entire coded packet, additional redundant information is incrementally transmitted if the decoding fails on the first attempt.
H-ARQ-type-III also belongs to the class of incremental redundancy ARQ schemes. However, with H-ARQ-type-III, each retransmission is self-decodable which is not the case with H-ARQ-type II. Chase combining (also called H-ARQ-type-III with one redundancy version) involves the retransmission by the transmitter of the same coded data packet. The decoder at the receiver combines these multiple copies of the transmitted packet weighted by the received signal to noise ratio (SNR). Diversity (time) gain is thus obtained. In the H-ARQ-type-III with multiple redundancy version different puncture bits are used in each retransmission.
The choice of H-ARQ mechanism however is important. Window based Selective Repeat (SR) is a common type of ARQ protocol employed by different systems including RLC R99. SR is generally insensitive to delay and has the favorable property of repeating only those frames that have been received in error. To accomplish this feat, the SR ARQ transmitter must employ a sequence number to identify each frame it sends. SR may fully utilize the available channel capacity by ensuring that the maximum frame sequence number (MBSN) exceeds the number of frames transmitted in one round trip feedback delay. The greater the feedback delay the larger the maximum sequence number must be. However, when Hybrid ARQ is partnered with SR, several difficulties are seen.
The memory requirements to the user equipment (UE) are high. The mobile must store soft samples for each transmission of a frame. The MSBN frames may be in transit at any time. A large MBSN requires significant storage in the UE adding to the costs of the unit.
Hybrid ARQ requires the receiver to reliably determine the sequence number of each transmission. Unlike conventional ARQ, every frame is used even if there is an error in the data. In addition, the sequence information must be very reliable to overcome whatever channel conditions have induced errors in the data. Typically a separate, strong code must be used to encode the sequence information, effectively multiplying the bandwidth required for signaling
Stop-and-wait is one of the simplest forms of ARQ requiring very little overhead. In stop-and-wait, the transmitter operates on the current frame until the frame has been received successfully. Protocol correctness is ensured with a simple one-bit sequence number that identifies the current or the next frame. As a result, the control overhead is minimal. Acknowledgement overhead is also minimal, as the indication of a successful/unsuccessful decoding (using acknowledged/not-acknowledged i.e. ACK, NACK, etc) may be signaled concisely with a single bit. Furthermore, because only a single frame is in transit at a time, memory requirements at the UE are also minimized. Therefore, HARQ using a stop-and-wait mechanism offers significant improvements by reducing the overall bandwidth required for signaling and the UE memory.
However, one major drawback exists: acknowledgements are not instantaneous and therefore after every transmission, the transmitter must wait to receive the acknowledgement prior to transmitting the next frame. This is a well-known problem with stop-and-wait ARQ. In the interim, the channel remains idle and system capacity goes wasted. In a slotted system, the feedback delay will waste at least half the system capacity while the transmitter is waiting for acknowledgments. As a result, at least every other timeslot must go idle even on an error free channel.
N-channel stop-and-wait Hybrid ARQ offers a solution by paralleling the stop-and-wait protocol and in effect running a separate instantiation of the Hybrid ARQ protocol when the channel is idle. As a result no system capacity goes wasted since one instance of the algorithm communicates a data frame on the forward link at the same time that the other communicates an acknowledgment on the reverse link. However, the receiver has to store a number of N frames for this scheme.
The physical layer (PHY) HARQ is being studied as a potential technique to support UTRA FDD Enhanced uplink. PHY HARQ allows a reduction of the transmission delay and an improvement of the system capacity. A user equipment (UE) communicates with one or more base stations (Node Bs), and the data packets (RLC SDU) from the UE are send to Node B frame by frame over an uplink. If a physical layer (PHY) frame from the UE is received correctly by a Node B which is connected with the UE, the Node B will send back an acknowledged (ACK) signaling to the UE over a downlink, and the UE will then transmit a new PHY frame. If the PHY frame is not received correctly by the Node B, a not-acknowledged (NACK) signaling will be sent to the UE, and the UE will retransmit the PHY frame again. PHY HARQ can be implemented by using an N-channel Stop-And-Wait (SAW) HARQ mechanism, in which the PHY frames are sent in N different continue time-multiplex channels. Each of the N channels has an independent HARQ processing, in other words, the error PHY frame only can be retransmitted at the next frame of the same channel which transmitted the original packet. Under such conditions, the PHY frame can be transmitted continuously, and system does not waste time to wait for the ACK/NACK signaling. Several key factors will decide about the selection of the sub-channel number, such as transmission delay, processing times of the UE and the Node B, the Node B buffer size as well as ACK/NACK signaling length. In the case of the sub-channel number is too small, such as 2, the delay time between original transmission and retransmission is short. However, in this case, UE and Node B have to process a PHY frame more quickly, and ACK/NACK singling transmission power may be higher, since short length ACK/NACK signaling has not enough processing gain. To avoid the problems described above, the sub-channel number is always selected 4 or 5 to insure that Node B and UE have enough processing time when e.g. a 10 millisecond (ms) transmission time interval (TTI) is used.
When a data packet (RLC SDU) needs to be transmitted in a physical layer (PHY) channel, it needs to be segmented, encapsulated step by step by the radio link control (RLC) layer and the media access control (MAC) layer at the transmitter side. The receiver receives the PHY frames carrying the information of the data packets (RLC SDU). The receiver RLC layer will deliver the data packet (RLC SDU) to the higher layer when all PHY frames are received successfully.
Since each physical frame only contains a part of the information of the data block (RLC SDU), any lost or delayed PHY frame will cause a total loss or delay of the data frame (RLC SDU). If any PHY frame is incorrect, under an N-channel SAW HARQ mechanism, the frame will be retransmitted at the next frame in the same sub-channel. If the incorrect frames are the last several frames belong to the data packet (RLC SDU), there it is likely not any other frames carrying information of the data packet (RLC SDU) that need to be transmitted in other sub-channels, when the incorrect frames are transmitted in one or more sub-channels. Thus the continuity of the data packet (RLC SDU) transmission is broken. The probability of the data packet (RLC SDU) with a longer transmission delay time will increase along with the retransmission probability and sub-channel number value N is increasing. This will decrease the transmission efficiency single data packets as well as increase the delay time. It is very harmful to the service of time sensitive transmission of small data packet (RLC SDU).
Under PHY HARQ, the block error rate (BLER) of the first transmission is always equal to 10%˜20% to get a HARQ gain. Conventionally, the BLER of the first transmission is same for all PHY frames carrying the information of the data packet (RLC SDU).
All the above approaches for communication and data transfer schemes have in common that they do not provide a solution for the unsatisfactory frame transfer termination characteristics of conventional HARQ techniques.
Therefore, it is desirable to have HARQ techniques that can overcome these problems and that provide improved frame transfer termination characteristics.